how to make straight edges smoother
I see at least 2 methods. The one you'll choose will depend on the amount of vertices you want and the time you want to spend on each object.

First method, bevels (faster but more vertices): You can select your object, go in Edit mode, select the angles you want, CtrlB to bevel, and set the bevel values in the operator panel. If the amount of Segments is low you can keep a rather low-poly version.

Second method, bevels + normal map (longer but no additional vertices): You can duplicate your object, bevel the copy, create a normal map from the beveled version, and use the normal map on the low-poly. This way you have zero additional vertex:


Create your low-poly version.





Duplicate it. Select the copy and in Edit mode, bevel the angles you want. Also, pump it up slightly with AltS (Shrink Fatten tool), keep pressing Shift if you want to be more precise. Give your object a Subdivision Surface modifier.





Bake the high-poly normals. To do so, first unwrap the low-poly (Smart UV Project mode), create a new black image in the UV Editor (32 Bit Float option activated I guess), give your low-poly a material with Image Texture > Normal Map > Diffuse > Output. Don't plug the Image Texture for the moment but switch it to Non-Color Data and load the image you've created before. Select the high poly object, then the low-poly, and in the Render > Bake panel, choose Bake Type > Normal, enable Selected to Active, click the Bake button. Set the Ray Distance at the right value, so that you have as less artefacts as possible. Anyway you'll necessary have some with this kind of geometry.





Export the normal map, import it in an image editor software, try to remove as many artefacts as possible, don't try it to make it perfect though, reimport the image, load it in the Image Texture node, plug the Image Texture node into the Normal socket of the Diffuse. You can also bring some corrections on your normal map directly in the 3D View / Texture Paint mode: Plug your Image Texture in the Color socket of the Diffuse to display it on the object, switch the 3D View to Texture shading, and use the Clone brush for example. When you're good, replug the Image Texture in the Normal Map node.